Spot instances and Spot Fleets

Amazon EC2 Spot instances: let you take advantage of unused EC2 capacity in the AWS cloud. Spot instances are available at up to a 90% discount compared to On- Demand prices. You can use Spot instances for various stateless, fault-tolerant, or flexible applications, such as big data, containerized workloads, CI/CD, web servers, high -performance computing (HPC), and other test and development workloads.

Usually Spot Instances will be used to systems where continuation  system is not required. termination notification of system will be notified with in 2 mins of time only.

Spot Blocks: To stop your spot instances from being terminated even if the spot price goes over your max spot price. You can set spot blocks for between one to six hours currently.

Spot Fleets:

Spot Fleets will try and match the target capacity with your price restraints.